You should check out this site http://www.transformerland.com
Their prices are good and selection is deep. They also have a great archive to help ID all those missing parts. What are you looking for, I have a bag of weapons from my old figures.
Yeah, I frequent Transformerland, but they just don't have what I am looking for. The following is what I am looking for:
Trypitcon: almost all the small parts
Weirdwolf: tail, gun, and headmaster robot
THunderwing: all 3 guns
Needlenose: tail fin
Ebay prices on this stuff is a little crazy.